Energy storage technologies have received more and more attention in recent years. As the applicability of energy storage technology is extended to mobile phones, camcorders, notebook PCs and even electric automobiles, there is a growing demand for high energy-density batteries as power sources for such electronic devices. As batteries capable of meeting this demand, lithium secondary batteries are considered the most promising batteries and are being actively researched.
Many secondary batteries are currently available. Of these, a typical example of the lithium secondary batteries developed in the early 1990's includes an anode made of a carbonaceous material capable of intercalating/deintercalating lithium ions, a cathode made of a lithium-containing oxide, and a non-aqueous electrolyte solution containing an appropriate amount of a lithium salt in a mixed organic solvent.
Non-aqueous electrolyte solutions containing organic solvents are prone to oxidation during long-term storage at high temperatures. This oxidation causes gas evolution and thus leads to the swelling of batteries, eventually resulting in degradation of the batteries. The gases arising from the decomposition of the electrolyte solutions may deform pouch or can type battery assemblies to cause internal short circuits. In extreme cases, the batteries may catch fire or explode. The oxidation of the electrolyte solutions may be accelerated by transition metals dissolved out under high voltage conditions.
In efforts to solve such problems, various additives have been proposed to prevent the swelling of batteries in non-aqueous electrolyte solutions. An example of such additives is a dinitrile compound having two or more ether bonds. The dinitrile compound is known to inhibit oxidation between an electrolyte solution and a cathode to suppress heat release. The dinitrile compound is also known to inhibit oxidative decomposition of an electrolyte solution to prevent a battery from swelling.
In a general method for the preparation of the dinitrile compound, a base catalyst, such as an alkali metal hydroxide or a quaternary ammonium compound, is used for a cyanoethylation reaction between an alcohol compound and acrylonitrile. The use of sodium hydroxide, which is most economically advantageous and is easy to synthesize, is widely known as the base catalyst.
However, water used as a mediator for the cyanoethylation reaction reacts with acrylonitrile to form cyanoethanol, which further reacts with acrylonitrile to form bis(2-cyanoethyl)ether, which acts as an impurity.
Some methods have been attempted to suppress the formation of the by-product, for example, by controlling the amount of acrylonitrile consumed in the reaction and by dropping acrylonitrile at a low rate to control the acrylonitrile concentration. However, these methods suffer from some problems, such as an increase in the amount of the alcohol compound remaining in the reaction mixture, causing low quality of the final product.
Another possible method is to prevent the occurrence of cyanoethylation due to the presence of water by reacting the reactants under non-aqueous conditions. In this case, however, the polymerization of acrylonitrile may occur depending on the presence of an alkali metal hydroxide or an organic base, leading to discoloration of the reactants.
On the other hand, a suggestion to solve such problems is described in Japanese Patent Registration No. 3946825, which discloses a method for preparing a cyanoethyl compound in the presence of lithium hydroxide as a reaction catalyst under anhydrous conditions. This method was reported to be useful in suppressing the formation of bis(2-cyanoethyl)ether and reducing the occurrence of coloring due to the polymerization of acrylonitrile.
However, lithium hydroxide is not effective in preventing the polymerization of the nitrile compound having an unsaturated bond and its relatively low solubility increases the reaction time.